Re: warning: dereferencing type-punned pointer

BTW, I tried the same experiment of building without
-fno-strict-aliasing using gcc 11.4.1 (from RHEL9).
I see one more warning than Tatsuo-san did:

In file included from verify_heapam.c:18:
verify_heapam.c: In function ‘check_tuple_attribute’:
../../src/include/access/toast_internals.h:37:11: warning: dereferencing type-punned pointer will break strict-aliasing rules [-Wstrict-aliasing]
37 | (((toast_compress_header *) (ptr))->tcinfo >> VARLENA_EXTSIZE_BITS)
| ~^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
verify_heapam.c:1693:24: note: in expansion of macro ‘TOAST_COMPRESS_METHOD’
1693 | cmid = TOAST_COMPRESS_METHOD(&toast_pointer);
| ^~~~~~~~~~~~~~~~~~~~~

This looks a bit messy to fix: we surely don't want to pierce
the abstraction TOAST_COMPRESS_METHOD provides. Perhaps
the toast_pointer local variable could be turned into a union
of struct varatt_external and toast_compress_header, but that
would impose a good deal of notational overhead on the rest
of this function.

The good news is that we get through check-world (although
I didn't try very many build options).

regards, tom lane